Has anyone wondered what the variation in FT4 is throughout the day and how great the fluctuations really are? I have heard doctors say it varies throughout the day so it's not a reliable or accurate test but it looks like TSH varies just as much so it seems contradictory to measure one believing it's a reliable test and dismiss the other. I wonder what this means for our blood tests?
The document states...
"A significant regular variation with a cycle-length of half an hour was found in TSH, free T3 and free T4. This rhythm accounted for a significant part of the total variation in the levels of TSH, free T3 and free T4. The mean amplitude of the short-term variation is 13, 15 and 11 per cent of the mean level of the respective hormones. The data suggest a pulsatile release of hormones from the thyroid gland governed by a pulsatile TSH secretion."
This freely accessible paper is somewhat more recent.
Standardised Resting Time Prior to Blood Sampling and Diurnal Variation Associated with Risk of Patient Misclassification: Results from Selected Biochemical Components
Thanks. The variation througout the day doesn't look like a huge up and down but more like a slow curve which would capture a reliable snapshot especially if blood tests are always at the same time of day. I presume FT4 follows a similar curve except there seems to be less diurnal variation?
Thanks, FT4 looks like the more reliable hormone to test because of less fluctuation. I'm wondering why docs think TSH only is necessary and why they think TSH is the most reliable test?
helvella i wonder why the TSH curve is so smooth but the other two are quite jagged do other things apart from TSH affect their release or is this just an artefact of them being used by the body? Or indeed due to another reason. Hope you can elucidate
The jaggedness is a reflection of the precision of the assays. If you have say an imprecision of 4%, there can be a difference up to 4% either way from one socalled "identical" reading to the next. This shows itself as jaggedness especially when the real changes are very small. However when as for TSH the changes are big, they swamp out the smaller imprecision of measurement, so the curves look to be smoother.
diogenes thanks for that illumination. After I posted I did think it might just be how it was measured and how precisely. It is obvious now you have explained it. Much appreciated 👍🏽
The short term variations in TSH, T3 and T4 are due to the pulsatile nature of TSH secretion. More importantly a lot of the variation will be due to the limited accuracy of the assays which have a margin of error of about 7% as I vaguely remember.
Another paper by amongst others the dreaded Weetman - but a sound paper for all that. Shows that all patients studied had a pulsatile TSH over a day but only a fraction of them had diurnal variation in FT4 and FT3, and the variation was small.
Free Triiodothyronine Has a Distinct Circadian Rhythm That Is Delayed but Parallels Thyrotropin Levels
W. Russell R. F. Harrison N. Smith K. Darzy S. Shalet A. P. Weetman R. J. Ross
The Journal of Clinical Endocrinology & Metabolism, Volume 93, Issue 6, 1 June 2008, Pages 2300–2306, doi.org/10.1210/jc.2007-2674
TSH varies more - up to 75% over the course of a day. A sudden rise in T4 or T3 after taking hormones is more likely to cause problems. TSH test is cheap and cheerful.
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